How come my skeleton no longer works after mirroring it's mesh?
Further to sambler's answer - the Mirror modifier has the option to mirror vertex groups, which helps significantly in avoiding this problem in the first place. For this to work, however, your groups need to be named with a suffix indicating what side of the figure they're on.

Vertex groups for left-side should end in the suffix .L, vertex groups for right-side should end in the suffix .R. This is not configurable, which irritates me :)

Both sets of groups need to exist, even if you don't have any vertexes in one half of them before adding the mirror modifier. The modifier won't create needed groups. This would also be a good feature.

I do plan on trying to tackle both those irritants once I wrap my head around more of how the internals work.